1. Technical Field
The present disclosure relates to analog-to-digital converters and, particularly, to continuous time sigma-delta analog-to-digital converters.
2. Description of the Related Art
Delta-Sigma (ΔΣ; or sigma-delta, ΣΔ) modulation is a technique which has found increasing use in a range of modern electronic components, such as analog-to-digital and digital-to-analog converters. FIG. 1 shows a conventional continuous time sigma-delta analog-to-digital converter provided with an integrator (consisting of an operational amplifier APA and a feedback capacitor CPA), a low pass filter FPA, a quantizer QPA, and a feedback path via a current mode digital-to-analog converter DACPA.
A low unit gain bandwidth or low slew rate of the operational amplifier employed by the integrator gives rise to distortion of the feedback signal. Particularly, in case of too low bandwidth the gain of the operational amplifier is too low to suppress the inherent non-linearity of its own transfer characteristic at higher frequencies, while in case of too low slew rate the gain of the operational amplifier becomes dependent on its input signal.
Due to the strong high pass shaping of the quantization noise in sigma-delta analog-to-digital converters, any distortion of the feedback signal results in high frequency noise being mixed with other high frequency noise and further being converted down to low frequency, thereby corrupting the noise shaping and degrading the signal to noise ratio (SNR). Hence for high signal to noise ratio and low distortion a very fast operational amplifier with high power dissipation is required.